XMODEM is a file transfer protocol which enables users to transmit files between different computers.
XMODEM was developed by Ward Christensen in 1977 and since then the method of sharing files has changed. It became extremely popular in the early bulletin board system (BBS) market because of its simple implementation. However it became inefficient as the modem speeds increased. The solution to this led to the development of several modified versions of XMODEM to improve the performance and resolve other issues with the file sharing protocol. XMODEM is the most modified computer program in history.
XMODEM breaks up the original data into a series of “packets” which are sent to the receiver. It also contains additional information which allows the receiver to determine whether that packet was correctly received or not. The packet is resent after requested by the receiver if an error encounters. The transfer is aborted because of a string of bad packets.
Following table displays the various symbols used during transfer and their meaning –
|SOH||Start of Header|
|EOT||End of Transmission|
|ETB||End of Transmission Block|
Packet Structure :
The original packet structure of XMODEM is given below –
Checksum is the sum of all bytes in the packet modulo 256. In other words it can be computed by discarding all but the eight least significant bits of the result of summation. In this way. For example, if this checksum method is used on a tiny data packet containing only two bytes carrying the values 130 and 130, the summation of these codes is 260, and the resulting checksum is 4. This ensures that the checksum is stored in 8 bits only.
Method of transfer :
Transfers are receiver-driven, i.e., the transmitter will not send any data until an initial NAK is sent by the receiver. The transmitter waits until the receiver sends a NAK byte. The NAK byte is the signal that the receiver is ready to start. This is the initial byte which displays that the file transfer is ready to begin. If the receiver takes too long or an error occurs then the transmitter will stop waiting or “Time Out”.
The file transfer will restart if it gets timed out. A NAK is also sent if the receiver does not receive a valid packet within ten seconds while still expecting data due to the lack of a EOT character.
Receiver performs the following tasks and transfers one packet at a time –
- It checks that the packet number sent matches the actual packet number i.e., 2nd byte is checked in the packet. In case of any discrepancy it sends a CAN byte to cancel the transfer.
- It adds the 1’s compliment and the packet number that is the 3rd and the 2nd bit to check if they add up to 255. In case of any discrepancy it sends a CAN byte to cancel the transfer.
- Packet’s checksum is calculated after retrieval and is compared to the one received from the sender at the end of the packet. If the condition SUM = CHECKSUM is true then the receiver sends an ACK byte to the transmitter which is an indication to send the next packet in sequence. If the condition is not satisfied then a NAK byte is sent which indicates the transmitter to again send the byte.
Transfer completion :
If the transmitter sends an EOT byte instead of a SOH byte, the receiver sends a NAK byte.
If the transmitter sends another EOT immediately after that,the receiver sends an ACK byte and the transfer is complete. The receiver has the ability to can cancel the transfer at any time by sending a CAN byte whereas the transmitter can only cancel between blocks by sending a CAN byte.
Attention reader! Don’t stop learning now. Get hold of all the important CS Theory concepts for SDE interviews with the CS Theory Course at a student-friendly price and become industry ready.
- Difference between File Transfer Protocol (FTP) and Secure File Transfer Protocol (SFTP)
- File Transfer Protocol (FTP) in Application Layer
- Simple Mail Transfer Protocol (SMTP)
- Principle Of Reliable Data Transfer Protocol
- Network News Transfer Protocol (NNTP)
- C program for file Transfer using UDP
- Hot Standby Router Protocol (HSRP) and Virtual Router Redundancy Protocol (VRRP)
- Difference between Stop and Wait protocol and Sliding Window protocol
- Difference between Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)
- Cisco Discovery Protocol (CDP) and Link Layer Discovery Protocol (LLDP) in Data Link Layer
- Difference between Border Gateway Protocol (BGP) and Routing Information Protocol (RIP)
- Asynchronous serial data transfer
- Asynchronous Transfer Mode (ATM) in Computer Network
- Reliable Data Transfer (RDT) 2.0
- Reliable Data Transfer (RDT) 1.0
- Various Types of Transfer Modes in HDLC
- Remote File System (RFS) in File Management
- Internet Control Message Protocol (ICMP)
- Sliding Window Protocol | Set 1 (Sender Side)
- Sliding Window Protocol | Set 2 (Receiver Side)
If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to firstname.lastname@example.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.